The present invention relates to the utilization of a co-propagating Raman amplifier in an optical communication system and, more particularly, to the utilization of a co-propagating Raman amplifier in a wavelength-division-multiplexed (WDM) optical communication system.
Raman amplifiers have played an important role in advancing optical communication systems, primarily as a result of being capable of both increasing the capacity of such systems (in terms of higher data rate and more channels) and the transmission distance of the systems. Currently, most Raman amplifiers use a counter-pumped configuration in which the pump and message signals propagate in opposite directions through the fiber amplifier. To date, there have been limited applications of a xe2x80x9cco-pumpedxe2x80x9d Raman amplifier where the pump and message signals propagate in the same direction through the fiber amplifier. The co-pumped architecture has been avoided due to the (presumed) increased presence of noise in the co-pumped amplifier, where noise is defined as xe2x80x9cpump-signal crosstalkxe2x80x9d (i.e., the noise originating from the pump being coupled to the message signals through Raman gain) and xe2x80x9csignal-pump-signal crosstalkxe2x80x9d (i.e., the encoded signal(s) impressing information to the same or different signal wavelength via the Raman process). The issue of pump-signal crosstalk has been addressed by the applicants in their co-pending application Ser. No. 60/186,797, filed Mar. 3, 2000. The ability to reduce pump-signal crosstalk using the methods disclosed in the co-pending application has lead to the ability to analyze and overcome the problems associated with signal-pump-signal crosstalk.
In general, Raman amplification is an extremely fast process, where the amplitude modulation of the encoded signal channels over a limited bandwidth is impressed upon the Raman pump. Thus, in this environment, even a perfectly xe2x80x9cquietxe2x80x9d pump (i.e., a pump without noise) will become noisy during Raman amplification. This noise on the pump may then be impressed upon other message signals through the process of Raman amplification. For the purposes of understanding the teaching of the present invention, this effect will be defined as xe2x80x9csignal-pump-signalxe2x80x9d crosstalk (hereinafter referred to as xe2x80x9cSPS crosstalk), since the crosstalk between the signal channels is mediated by the pump. See, for example, the article entitled xe2x80x9cCross talk in Fiber Raman Amplification for WDM Systemsxe2x80x9d, by W. Jiang et al., appearing in the Journal of Lightwave Technology, Vol. 7, No. 9, 1989 at app. 1407-1411. In this theoretical paper, the crosstalk between two channels in a Raman amplifier was calculated. The crosstalk, even in the linear amplifier range (i.e., pump non-depletion), was shown to be severe for the co-propagating configuration. In its conclusion, the Jiang et al. reference stated that a counter-propagating arrangement would be preferred, since as the pump travels against the message signal, a stronger averaging effect exists, reducing the crosstalk.
It is also known in the art that SPS crosstalk depends on the modulation frequency of the channels, due to the relative propagation speed difference (i.e., group velocities) between the pump and message signals, as well as between the signals themselves. Such relative propagation speed difference introduces the walk-off of information in time, therefore effectively averaging the SPS crosstalk for higher frequencies. Such an effect results in a limited crosstalk bandwidth over which the SPS crosstalk may occur, which is much smaller as compared to the electrical bandwidth of the data. The crosstalk bandwidth in the counter-propagating configuration is known to be relatively small when compared to that encountered in the co-propagating configuration. See, for example, an article entitled xe2x80x9cBandwidth of cross talk in Raman amplifiersxe2x80x9d by F. Forghieri et al. appearing in the OFC ""94 Technical Digest at page 294. In this paper, the crosstalk bandwidth was defined as the frequency bandwidth in which the modulation depth onto the second continuous-wave signal channel is more than xe2x88x9220 dB. In this definition, the crosstalk bandwidth is determined by the fiber dispersion parameters at the wavelengths of the pump and signals, as well as the amount of Raman gain and pump depletion. In their study, the crosstalk bandwidth was determined to be approximately 100 MHz for a co-pumped Raman amplifier, as compared to only a 10 kHz bandwidth for a counter-pumped arrangement. Therefore, the Forghieri et al. paper concludes that the performance of an intensity modulated WDM system using a co-pumped Raman amplifier was severely limited by SPS crosstalk, noting as preferable, then, the counter-propagating configuration. A similar conclusion was made in the article xe2x80x9cCrosstalk due to stimulated Raman scattering in single-mode fibers for optical communication in wavelength division multiplex systemsxe2x80x9d by H. F. Mahlein appearing in Optical and Quantum Electronics 16, (1984), p. 409 et seq.
It is well-known by those skilled in the art that the term xe2x80x9crelative intensity noisexe2x80x9d (RIN) is often used to characterize fluctuations in photocurrents. RIN is defined in terms of detected electrical power as the power-spectral density of the photocurrent in a 1xe2x80x94Hz bandwidth at a specified frequency divided by the average power of the photocurrent. Although the term xe2x80x9cRINxe2x80x9d indicates that this quantity is usually used to characterize fluctuations arising from noise, the same quantity can be used to characterize fluctuations due to signal modulation, as is the case here.
As mentioned above, SPS crosstalk depends on both pump depletion and Raman gain. It is known by those skilled in the art that the amount of Raman gain is known as the xe2x80x9con/offxe2x80x9d gain, and defined as ratio of output signal power in the presence of the Raman pump to the output signal power in the absence of the Raman pump. If a Raman pump amplifies an optical signal to an optical power comparable to the power of the Raman pump, the Raman pump will experience xe2x80x9cpump depletionxe2x80x9d. This means that the power of the pump, at some position within the amplifier, will become significantly less than it would be in the absence of the signal, and that the xe2x80x9cnetxe2x80x9d Raman amplification will be reduced. In practice, pump depletion is measured at the output of the amplifier and is defined as the intensity difference of the pump output xe2x80x9cwithxe2x80x9d and xe2x80x9cwithoutxe2x80x9d signal channels. In general, the smaller the pump depletion and Raman gain, the smaller the SPS crosstalk. Therefore, it is expected from the prior art studies that co-pumped Raman amplifiers are limited to applications with small levels of Raman gain and pump depletion. See, for example, the article entitled xe2x80x9cWide-Bandwidth and Long-Distance WDM Transmission using Highly Gain-Flattened Hybrid Amplifierxe2x80x9d by S. Kawai et al., appearing in IEEE Photonics Technology Letters, Vol. 11, No. 7, Jul. 1999. In this article, 4 dB of Raman gain in a co-propagating-pump geometry was used in a discrete Raman amplifier as a part of the gain-flattened hybrid amplifier.
The prior art is replete with references describing the SPS crosstalk problem present in fiber Raman amplifiers and the utilization of a counter-propagating amplifier configuration to overcome this problem. The Jiang et al. article referenced above describes a solution involving limiting the signal gain and the injected pump power to values well below the threshold for Raman amplification. The Forghieri et al. and Mahlein articles suggest a solution in terms of eliminating all components within the crosstalk bandwidth (for example, 100 MHz), which is not practical in WDM applications. Several papers have stated that a counter-propagating configuration eliminates SPS crosstalk through averaging. For example, see an article by S. A. E. Lewis et al. appearing in Electronic Letters, Vol. 35, No. 11, 1999, at page 923. To date, therefore, most Raman amplifiers have been limited to the counter-propagating configuration.
However, as WDM optical communication systems continue to grow in capacity and reach longer distances, more system benefits could be realized from co-propagating Raman amplification. For example, a co-propagating Raman amplifier would allow for bidirectional pumping of a Raman amplifier, as well as bi-directional transmission of the message signals. Also, the aggregate signal powers have reached magnitudes that can easily deplete a Raman pump.
Thus, a need remains in the art for an arrangement capable of providing significant co-propagating Raman amplification in a WDM optical transmission system.
The need remaining in the prior art is addressed by the present invention, which relates to the utilization of a co-propagating Raman amplifier in optical communication system and, more particularly, to the utilization of a co-propagating Raman amplifier exhibiting reduced signal-pump-signal (SPS) crosstalk in a wavelength-division-multiplexed (WDM) optical communication system.
In accordance with the present invention, it has been discovered that co-propagating Raman amplification, when utilized into the pump depletion region, can be used in virtually any WDM optical transmission system, as long as the signal channels interacting with any given Raman pump exhibit small integrated RIN values over the fiber crosstalk bandwidth. In particular, it can be shown that SPS crosstalk in co-propagating fiber Raman amplifiers can be significantly reduced by altering the properties of the input signals so as to minimize the integrated RIN values of the signal channels over the fiber crosstalk bandwidth.
In accordance with the present invention, one or more of the following methods is used to provide the desired reduction of the integrated RIN values of the signal channels interacting with any given co-propagating Raman pump in the fiber crosstalk bandwidth: (1) transmission of a sufficient number of independent channels (using xe2x80x9cdummyxe2x80x9d channels, if necessary); (2) encoding the data such that any given signal channel has a small ratio of integrated RIN values over the fiber crosstalk bandwidth to that over the entire signal electrical bandwidth; and/or (3) decorrelating a plurality of input signals to reduce the integrated RIN values over the fiber crosstalk bandwidth. In general, the purpose of the present invention is to reduce the integrated RIN values over the fiber crosstalk bandwidth so as to essentially eliminate problems associated with SPS crosstalk in a co-propagating Raman amplifier.
In a preferred embodiment of the present invention, all three of these concepts would be used to provide for the greatest reduction in SPS crosstalk. However, it has been found that the implementation of even one of these features is sufficient to provide for a viable commercial WDM system using a co-propagating fiber Raman amplifier.
Various and further features and aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.